1. Field of the Invention
This invention relates to a method of manufacturing a catalytic converter for the purification of exhaust gas arranged in an exhaust system of an internal combustion engine, and more particularly to a method of manufacturing a catalytic converter arranged on the way of an exhaust pipe or an exhaust manifold in an exhaust system of a vehicle such as automobiles or the like to conduct the purification of exhaust gas.
2. Description of Related Art
In general, the catalytic converter for the purification of exhaust gas used in the exhaust system of the internal combustion engine comprises a catalyst carrier provided with a catalyst such as platinum or the like, a heat-insulating seal layer covering the outer surface of the catalyst carrier, and a metal shell housing an assembly of the carrier and the seal layer therein.
As the catalyst carrier in such a catalytic converter, there is used a monolithic carrier having a honeycomb shape at section and made from cordierite or the like.
As the heat-insulating seal layer, there is mainly used an inorganic fiber sheet. This sheet is used for holding the catalyst carrier at an adequate temperature to effectively develop the catalytic action and for preventing the damage of the catalyst carrier due to the contact with the outer metal shell during the running of the vehicle or the like, or preventing the leakage of exhaust gas from a space between the metal shell and the catalyst carrier.
As a method of manufacturing the catalytic converter, there has usually and widely been adopted so-called cram-shell system wherein the heat-insulating seal layer comprised of the inorganic fiber sheet is first wound around the catalyst carrier and then assembled into the metal shell by sandwiching between previously two divided parts of the shell. Recently, there is proposed a method wherein a cylindrical metal shell is used instead of the two-divided metal shell and the catalyst carrier covered with the heat-insulating seal layer is directly inserted into the cylindrical metal shell from a viewpoint of the improvement of operability and the like.
In order to closely fix the catalyst carrier to the inside of the metal shell through the heat-insulating seal layer when the catalyst carrier covered with the heat-insulating seal layer is directly inserted into the cylindrical metal shell, JP-A-7-189677 and JP-A-7-189678 propose a method of facilitating the assembling of the catalyst carrier to the metal shell wherein the catalyst carrier covered with the inorganic fiber sheet as the heat-insulating seal layer is placed in a bag of an airtight film and then the inside of the bag is evacuated to reduce the thickness of the inorganic fiber sheet, or JP-A-57-146954 and JP-A-59-126023 propose a method wherein the inorganic fiber sheet is closely sealed in an airtight film bag and then wound around the catalyst carrier.
As the inorganic fiber sheet, there has mainly been used an inorganic fiber sheet having an excellent handling property, which is comprised of a mixture of ceramic fibers and vermiculite being relatively poor in the heat resistance and having a high-temperature swelling property. Recently, the temperature of exhaust gas rises with the remarkable improvement of engine performances for automobiles and hence the inorganic fiber sheet used as the heat-insulating seal layer is required to have a higher heat resistance. For this end, it is now expected to use a sheet made of crystalline alumina fiber having an excellent heat resistance instead of the conventionally used inorganic fiber sheet made of the mixture of vermiculite and ceramic fibers.
However, the crystalline alumina fiber sheet is excellent in the heat resistance as compared with the inorganic fiber sheet made of the vermiculite and ceramic fiber mixture, but has drawbacks that it has not a high-temperature swelling property as in vermiculite and is relatively low in the fiber strength and the crystalline alumina fiber sheet is apt to broken under an influence of pulsation of the exhaust gas. Therefore, it is required to improve the sealing property by winding the crystalline alumina fiber sheet around the catalyst carrier at a state of increasing the bulk density of the sheet. In the conventional method of increasing the bulk density by reducing the thickness of the inorganic fiber sheet through the evacuation inside the airtight film bag, however, an external pressure is an atmospheric pressure and the bulk density can not sufficiently be increased and hence it is difficult to obtain the crystalline alumina fiber sheet at a state of increasing the bulk density by such a conventional method. Moreover, a method of closely fixing the catalyst carrier to the inside of the metal shell through a heat-insulating seal layer having a high compression restoring property is not known up to the present at all.